1. Field of the Invention
This invention relates to microfine mixed fibers spun according to melt-blown process, a fiber product consisting of the microfine mixed fibers and a process for producing the fibers and the fiber product. More particularly, it relates to fiber products consisting of microfine mixed fibers consisting of high melting point microfine fibers and low melting point microfine fibers, the melting point difference between both the fibers being 10.degree. C. or more, and the content of the low melting point microfine fibers in the mixed fibers being 10 to 90% by weight, and a process for producing the fiber products.
Concretely, it relates to a cylindrical filter having a good pressure resistance strength and filtration accuracy and a long filtration life.
2. Description of the Related Art
Microfine fibers have been processed into non-woven fabrics or molded products and have been broadly used as the surface material for disposable diaper, dust-proof clothes, mask, wiping cloth, filter for microfiltration, etc. As the filter for microfiltration, an air filter, a filter for washing liquid during the steps of producing electronic equipments, a prefilter for water for pharmaceutical preparations, a filter for removing microorganisms from foods or potable water, etc. are known.
Japanese patent application laid-open No. Sho 54-134177 discloses a process for producing a non-woven fabric of microfine fibers wherein a thermoplastic resin is degraded to have a low viscosity in a spinning device, followed by spinning according to melt-blown process. Further, Japanese patent application laid-open No. Sho 60-99057 discloses a process for producing a non-woven fabric of microfine conjugate fibers wherein two kinds of thermoplastic resins are conjugated into side by side type, followed by spinning according to melt-blown process.
As the filter, Japanese patent application laid-open No. Sho 60-216818 discloses a filter for microfiltration obtained by stacking microfine fibers on a shaft and supporting the structure only by way of mechanical entanglement of the fibers to one another, and having a constant void ratio. Japanese patent application laid-open No. Hei 1-297113 discloses a cylindrical filter obtained by winding up non-woven fabrics, wherein the closer to the outer surface of the filter, the larger the fiber diameter and the pore diameter. Japanese patent application laid-open No. Hei 4-126508 discloses a cartridge filter consisting of microfine conjugate fibers, prepared according to melt-blown process. Japanese patent application laid-open No. Hei 5-96110 discloses a cartridge filter consisting of microfine fibers obtained according to melt-blown process, wherein the fiber diameter is successively varied.
In the case of conventional non-woven fabrics consisting of microfine fibers of a single component, as the structure have been supported mainly by entanglement of fibers with one another, drawbacks have been raised that the non-woven fabrics have a low strength and many fluffs, and are unsuitable to apply for the surface material for disposable diaper. When the fabrics are heat-treated by means of heating rolls or the like, in order to enhance the strength of the fabrics, then the fibers are liable to melt and the resulting non-woven fabrics become film-like and has an inferior feeling.
Further, in the case of conventional filters using microfine fibers of a single component, adhesion between fibers is few. Thus, drawbacks have been raised that the filtration accuracy varies due to heat-sterilizing treatment, high temperature filtration, vibration or the like, the hardness is low and the withstanding pressure is insufficient.
Further, the conjugate melt-blown process has drawbacks that the spinneret is complicated and expensive, and it is difficult to control the melt viscosity for uniformly feeding different kinds of polymers into the respective extrusion holes.